A Personal Nanoparticle Counter This proposal addresses the need for a rugged, wearable instrument for the precise, real-time monitoring of airborne number concentrations for fine, ultrafine and nanometer-sized particles. Proposed is a new type of laminar-flow water condensation method that detects and counts particles as small as 5 nm in a tippable device. We aim to demonstrate the feasibility of this technology for a portable and potentially wearable device, to measure nanometer and ultrafine particle concentrations with 1-s time resolution. Additionally, we will demonstrate its suitability for long term monitoring when power source is available. Specific aims are: 1) to verify the feasibility of a miniature, water-based condensation particle counter capable of: detecting at least 50% of particles with a diameter of 6 nm; detecting at least 95% of those particles with diameters above 10 nm; measuring particle concentrations from 103 - 106 cm-3; providing 1-sec time resolution at ambient concentrations with statistical error of less than 5%; providing comparable data, within 15%, to bench top condensation particle counters for ambient air 2) to demonstrate operation in any orientation, and when subject to jarring and jostling typical of what to be expected when worn. 3) to achieve the above for less than 2 watts of power consumption by the condensational growth cell and optics, with a weight of less than 200 grams (ie without control electronics or battery).